A fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to negative oxygen ion. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing electrons, water and also, for example, carbon dioxide (CO2). Anode 100 and cathode 102 are connected through an external electric circuit 111 having a load 110 for the fuel cell withdrawing electrical energy alongside heat out of the system. The fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are shown below:Anode: CH4+H2O=CO+3H2 CO+H2O=CO2+H2 H2+O2−=H2O+2e−Cathode: O2+4e−=2O2−Net reactions: CH4+2O2═CO2+2H2OCO+½O2═CO2 H2+½O2═H2O
In electrolysis operating mode (solid oxide electrolyzer cells (SOEC)) the reaction is reversed, i.e. heat, as well as electrical energy from a source 110, are supplied to the cell where water and often also carbon dioxide are reduced in the cathode side forming oxygen ions, which move through the electrolyte material to the anode side where oxygen oxidation reaction takes place. It is possible to use the same solid electrolyte cell in both SOFC and SOEC modes.
Solid oxide electrolyser cells operate at temperatures which allow high temperature electrolysis reaction to take place, the temperatures being for example, between 500-1000° C., but even over 1000° C. temperatures may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below:Cathode: H2O+2e−→2H2+O2−Anode: O2−→½2O+2e−Net Reaction: H2O→H2+½O2 
In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer (SOE) stacks the flow direction of the cathode gas relative to the anode gas internally in each cell as well as the flow directions of the gases between adjacent cells, are combined through different cell layers of the stack. Further, the cathode gas or the anode gas or both can pass through more than one cell before it is exhausted and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack.
SOFC and SOEC stacks include stacked cell elements and separators in a sandwiched manner wherein each cell element is constituted by sandwiching an electrolyte, the anode side and the cathode side. The reactants are guided by flow field plates to the porous electrodes.
A SOFC delivers in normal operation a voltage of approximately 0.8V. To increase the total voltage output, the fuel cells can be assembled in stacks in which the fuel cells are electrically connected via flow field plates (also: separator plates, interconnect plates, interconnector plates, bipolar plates, current collector plates). The desired level of voltage determines the number of cells needed.
Bipolar plates separate the anode and cathode sides of adjacent cell units and at the same time enable electron conduction between anode and cathode. Interconnects, or bipolar plates are normally provided with a plurality of channels for the passage of fuel gas on one side of an interconnect plate and oxidant gas on the other side. The flow direction of the fuel gas is defined as the substantial direction from the fuel inlet portion to the fuel outlet portion of a cell unit. Likewise, the flow direction of the oxidant gas, the cathode gas, is defined as the substantial direction from the cathode inlet portion to the cathode outlet portion of a cell unit.
Known cells are stacked one on top of each other with a complete overlap resulting in a stack with for instance co-flow having all fuel and oxidant inlets on one side of the stack and all fuel and oxidant outlets on the opposite side. One feature affecting the temperatures of the structure in operation is steam reformation of the fuel that is fed into the cell. Steam reformation is endothermic reaction and cools the fuel inlet edge of the cell. Due to the exothermicity of the electrochemical process, the outlet gases leave at higher temperature than the inlet temperature. When endothermic and exothermic reactions are combined in an SOFC stack a significant temperature gradient across the stack is generated. Large thermal gradients induce thermal stresses in the stack which are highly undesirable and they entail differences in current density and electrical resistance. Therefore the issue of thermal management of an SOFC stack exists: to reduce thermal gradients enough to avoid unacceptable stresses and to maximize electric efficiency through homogenous current density profile.
It is often desirable to protectively coat the flow field plates in order to slow down corrosion of the metal. Generally there are three main corrosion mechanisms that cause aging to solid oxide fuel cells and electrolyzers. The first is the formation of an oxide layer or layers that has low electrical conductivity onto the metal surface, another being the settling of chrome compounds evaporating from metal onto the active surfaces of the unit cell and reaction with electrochemically active materials weakening the electrochemical, chemical, electrical conductivity and/or gas permeability properties of the active material and the last being change of the bulk metal composition either by depletion of at least one bulk metal compound such as chromium or inward transfer of a compound not originally present in the bulk metal such as nickel from the fuel electrode or compounds from sealing solution. Oxide structures are generally used as protective coatings that on one hand slow down oxidant diffusion onto the surface of the metal and on the other hand diffusion of alloy atoms and compounds through the oxide structure. The price of the protective coating is often significant within the total costs of the cell stack and cost of the protective coating is on one hand influenced by the fabrication process used for the protective coating, the material and the surface to be coated protectively. Additionally it is not preferable to extend the electrically conducting protective coating to areas, which are used to seal the cell stack, because glass, ceramic materials or minerals generally used as sealants can react with the protective coating causing aging effects to the cell stack structures, for example because of increased gas leakages and/or increased electric conductivity. On the other hand, protective coatings having lower electrical conductivity values such as aluminium oxide compounds can be used to prevent chemical reactions between the steel structure and sealing materials as well as to prevent chromium evaporation from these areas.
The state of the art interconnect structures are made with forming processes from sheet metal plates. The maximum formability of the metal is limited by its mechanical properties and often both the channel area and the contact surfaces are not optimal as described above. Because of the limitation associated to the forming processes, interconnect plate structures either cause major pressure loss characteristics and/or the contact surface limits the electron transfer in the fuel cell both causing restrictions to the duty ratio of fuel cell or electrolyzer stack.
The performance of the protective coatings in SOFC interconnects is strongly linked to the chemical composition and microstructure of the deposited coatings. Since degradation of the cathode is a consequence of reactions between the cathode materials and volatile Cr oxide and oxyhydroxides, i.e., CrO3, CrO2 (OH)2 and CrO2(OH), which are transported through the gas phase on the active electrode, these coatings should have a dense microstructure. A dense microstructure is essential in harsh environments (high humidity and operating temperature) in order to decrease the growth of Cr-rich oxide scale, which may lead to uncontrolled breakaway oxidation and/or increase the ohmic resistance of the substrate-coating systems. The high H2O partial pressure on the anode side of a SOFC also inhibits the growth of Cr-rich oxide scale with low electrical conductivity. In addition, the anode electrode can be in direct contact with the interconnect structure, which in known SOFC operation conditions leads to transfer of nickel to the steel structure. This eventually changes the ferritic steel structure into austenitic structure having for example higher thermal expansion coefficient, electrical and mechanical properties compared to ferritic structure.
Protective metal oxide coatings can be manufactured by using various thermal spraying techniques. Prior to the spraying process, several material synthesis and powder manufacturing phases are used in order to form suitable feedstock for the spraying process, which are always required when conventional thermal spraying techniques (dry powder spraying) are employed. Some commonly used thermal spraying techniques are atmospheric plasma spraying (APS) and high velocity oxy-fuel spraying (HVOF). In general, material synthesis and powder manufacturing phases are specifically tailored according to raw material and spray equipment. As a result, time consuming research and development practises are needed in order to find the optimal material process routes. Secondly, the spray process should be optimized so that the coatings with favourable microstructure can be manufactured.
Since the metal oxide coatings used in SOFC and SOEC are relatively thin (<50 μm), the process steps, described previously, should be as robust as possible in order that any sort of variation in coating thickness and in microstructure can be minimized. Controlling the overall process becomes more challenging when conventional thermal spraying processes are being used. This is due to reason that the total coating process; including material and powder synthesis contain the multiple process steps which have a huge impact on the quality of the as-sprayed coating. This can be even more problematic when multi-layered coating structures are deposited, for example, protective metal oxide coating and another metal oxide contact layer, because the process steps are multiplied by the number of the deposited coating layers.
However, by using a novel thermal spraying technique such as the solution precursor thermal spraying (SPTS), where the feedstock is in form liquid and not in dry powder, the number of the process steps can be notably decreased. This is due to reason that the known material and powder synthesis phased are no longer needed. As a result, when then the number of process steps are decreased, the overall process becomes more robust and controllable. In addition, the coating deposition becomes more cost effective, which is more favourable when producing coatings for SOFC and SOEC stacks.
Document WO 96/288255 A1 discloses spray pyrolysis embodiments, in which no thermal flame is utilized in the spraying process. Thus, in WO 96/288255 the sprayed surface coating involves the series of separate thermal processing phases called calcination and sintering steps at 100-600° C. These process steps can involve a long processing time. Another drawback is that due to the long processing time a CrO2 layer has enough time to be formed. Since, solution is sprayed on a pre-heated substrate, the formation of cracks cannot be avoided due to evaporation of volatile compounds (solvents). As a result the adequate protection against Cr-migration cannot be achieved.